The use of Computer Assisted Surgery (CAS) systems for orthopedic operations in general, and for Total Knee Replacement (TKR) surgery in particular, is becoming increasingly more commonplace with advancements in CAS systems and equipment that ensure improved accuracy, fail safe operation and increasing ease of use.
During TKR surgery, the accuracy of cuts made in the patient's bone prior to the installation of any implant, and particularly the resection cut made in the condyles at the distal end of the femur, is important and contributes significantly to the success of the total knee replacement.
TKR surgery requires several precise cuts to be made in the femur and tibia, including the femoral condyle distal resection cut mentioned above, such that the implant fits correctly and best replicates the geometry of a natural healthy knee. Generally, in TKR surgery, the distal ends of both the femur and tibia are completely resected such that these joint articulation surfaces can be replaced with prosthetic implants. To perform these resection cuts, in both conventional and CAS TKR, it is well known to use a cutting guide which assists the surgeon such that he or she can make the resection cuts in the right position and orientation to ensure that the geometry of the knee, once reconstructed with the prosthetic implants, approaches as much as possible that of a healthy knee. In many conventional, i.e. non computer assisted, total knee replacement surgery a distal cutting block is positioned and aligned by the surgeon and pinned in place (typically on the anterior surface of the femur) such that the cutting slot is aligned in the correct location for the distal cut. In many CAS total knee replacement surgeries, it is also known to use a distal pin drill guide to accurately create the pin holes into which locating pins are inserted and employed to fix the distal cutting guide, either integrally formed with the distal pin drill guide or being a separate element, in the correct location to make the distal cut in the femoral condyles. Generally, the distal drill/cutting guide member comprises part of an assembly including an anterior guiding platform, that is fixed relative to the femur and on which the drill/cutting guide is displaceable by a selected, measurable amount to locate the drill/cutting guide in a predetermined position relative to the anterior guiding platform and therefore relative to the distal end of the femur. A tracked guide block is often intramedullarly fastened to the femur, and the anterior guiding platform can then be engaged thereto. Depending on the type of implant being used, and once aligned with the most distal femoral condyle, the drill/cutting guide can then be proximally displaced on the fixed anterior guiding platform by a selected amount corresponding to the amount of bone to be resected.
However, as TKR CAS techniques have developed, a large guide block which actually abuts the distal end of the femoral condyles is now less commonly used, and therefore alternate means are required to determine the exact location of the most distal point(s) on the femoral condyles, such that this can be used as a reference point from which the predetermined resection distance can be measured, calculated or otherwise determine, either by the surgeon and/or by the CAS system. Therefore, it is important to the success of the TKR procedure to be able to accurately determine the exact location of the most distal point on the femoral condyles, such that the distal resection distance can be accurately measured from this point.
The distal surfaces of the femoral condyles have been more recently located by CAS systems by using a digitizing pointer, which is calibrated and tracked in space by the CAS system and which is used to digitize a plurality of points on the surfaces of the condyles. The CAS system then uses these points, which are assumed to be representative of the distal condyle surfaces, to plot the condyle surfaces and thus determine the location of the most distal point thereof.
Several disadvantages exist with this method of determining the distal surface in general, and the most distal point thereon in particular. For example, such digitizing pointers typically have a relatively sharply pointed tip which can unintentionally penetrate a soft or damaged bone surface or any soft tissue or cartilage which may be present on the bone surface, depending on the amount of pressure exerted by the surgeon. While such measurement inaccuracies can be very small (such as 1 mm or less for example), these can nonetheless lead to significant differences between the actual distal surface of the condyles and that determined by the CAS system, resulting in improperly located distal resection cuts and thus proper fit of the implant. Additionally, while several points are taken on the distal condyle surfaces in order for the CAS system to digitize these surfaces, if the points chosen are not representative of the entire surface of if they are localized in one portion of the bone, errors in the resultant digitized surface can result. Determining by eye which portions of the condyle may be the most distal points, such that they can be digitized using the pointer, can also be particularly difficult on severely deformed or damaged bones.
Therefore, there is a need to provide a CAS system and device which enables a more accurate and simplified measurement of the most distal point(s) on the condyles of a femur prior to knee surgery.